Sex-linked and Mitochondrial Inheritance (Learning ...
Transcript of Sex-linked and Mitochondrial Inheritance (Learning ...
Sex-linked and Mitochondrial Inheritance (Learning Objectives)
• Explain how gender is determined in mammals. • Define X- or Y-linked genes. How does the location of a gene on the
X chromosome affect its gender-related transmission? • Use a Punnett square to determine the probability of passing of an X-
linked gene and the phenotype to girls or boys based on the genotypes of the parents.
• Explain the difference between sex-limited traits and sex-influenced traits.
• Explain X-inactivation and why it exists only in cells of females.• Explain the functions of the Y chromosome gene and the pattern of
inheritance of Y-linked traits.• Explain the pattern of inheritance of genes present on the
mitochondrial DNA.
Gender• Maleness or femaleness is determined at
conception
• Another level of sexual identity comes from the control that hormones exert on development
• Finally, both psychological and sociological components influence sexual feelings
Figure 6.1Figure 6.1
During the fifth week of prenatal development, all embryos develop two sets of:- Unspecialized (indifferent)
gonads - Reproductive ducts:
Müllerian (female-specific) & Wolffian (male-specific)
An embryo develops as a male or female based on the absence or presence of the Y chromosome
- Specifically the SRY gene (sex-determining region of the Y chromosome)
Sex determination in Mammals:the X-Y system
Karyotype designation: 46, XY (male)
46, XX (female) (homogametic)
Males are heterogameticGerm cells in testes (XY) produce sperms with
X: 50% Y: 50%
Females are homogameticGerm cells in ovaries (XX) produce only
X eggs
Figure 6.6
Sex Determination in Humans
Figure 6.6
X and Y ChromosomesX chromosome
- Contains > 1,500 genes- Larger than the Y chromosome- Acts as a homolog to Y in males
Y chromosome- Contains 231 genes- Many repeated DNA segments
Figure 6.2
Anatomy of the Y Chromosome
Figure 6.3
Pseudoautosomal regions (PAR1 and PAR2)- 5% of the chromosome- Contains genes shared with X chromosome
Male specific region (MSY) - 95% of the chromosome- Contains majority of genes including SRY and AZF (needed for sperm production)
SRY Gene• Encodes a transcription factor protein• Controls the expression of other genes• Stimulates male development• Developing testes secrete anti-Mullerian
hormone and destroy female structures• Testosterone and dihydrotesterone (DHT)
hormones are secreted and stimulate male structures
The inheritance of genes of X chromosome
• males have only a single X chromosome • almost all the genes on the X have no
counterpart on the Y• Genes are described as sex-linked or X-
linked.
X-linked Traits
Possible genotypesX+X+ −Homozyogus wild-type femaleX+Xm −Heterozygous female carrierXmXm −Homozygous mutant female
X+Y − Hemizygous wild-type maleXmY− Hemizygous mutant male
X-linked Recessive Traits
Examples:- Ichthyosis = Deficiency of an enzyme that removes cholesterol from skin
- Color-blindness = Inability to see red and green colors http://www.biology.arizona.edu/human_bio/problem_sets/color_blindness/color_blindness.html
- Hemophilia = Disorder of blood-clotting http://www.ygyh.org
Figure 6.7Figure 6.7
Ichthyosis
X-linked Dominant Traits
Congenital generalized hypertrichosis
Figure 6.10
Sex-Limited TraitsTraits that affect a structure or function occurring
only in one sex
The gene may be autosomal or X-linked
Examples:- Beard growth- Milk production- Preeclampsia in pregnancy
Sex-Influenced TraitsTraits in which the phenotype expressed by a
heterozygote is influenced by sexAllele is dominant in one sex but recessive in the
otherThe gene may be autosomal or X-linked
Example:- Pattern baldness in humans (autosomal)
- A heterozygous male is bald, but a heterozygous female is not
X Inactivation
Females have two alleles for X chromosome genes but males have only one
In mammals, X inactivation balances this inequality and one X chromosome is randomly inactivated in each cell
The inactivated X chromosome is called a Barr body
X Inactivation
X inactivation occurs early in prenatal development
It is an example of an epigenetic change
The XIST gene on the inactive X encodes an RNA that binds to and inactivates the X chromosome
Figure 6.12Figure 6.11
X InactivationA female that expresses the phenotype
corresponding to an X-linked gene is a manifesting heterozygote (calico cats)
Figure 6.12
Y-linked genes
The Y chromosome in males has 231 gene genes whose protein products are involved in:
a. control of changing sex of the fetus from female to male
b. development of male testesc. male fertility
http://ghr.nlm.nih.gov/chromosome=Y
Genomic Imprinting
The phenotype of an individual differs depending on the gene’s parental origin
Genes are imprinted by an epigenetic event: DNA methylation- Methyl (CH3) groups bind to DNA and suppress gene expression in a pattern determined by the individual’s sex
Imprints are erased during meiosis- Then reinstituted according to the sex of the individual
Figure 6.13
Mitochondrion• Organelle providing cellular energy
• Contains small circular DNA called mtDNA- 37 genes without noncoding sequences
• No crossing over and little DNA repair
• High exposure to free radicals
• Mutation rate is greater than nuclear DNA
• A cell typically has thousands of mitochondria, and each has numerous copies of its “mini-chromosome”
Figure 5.8
Mitochondrion• Mitochondrial genes are transmitted from
mother to all of her offspring
Figure 5.7
Mitochondrial DisordersMitochondrial genes encode proteins that participate in
protein synthesis and energy production
Several diseases result from mutations in mtDNAaternallyinherited
Examples:- Mitochondrial myopathies – Weak and flaccid muscles- Leber optical atrophy – Impaired vision
Ooplasmic transfer technique can enable woman to avoid transmitting a mitochondrial disorder
Heteroplasmy
Figure 5.9
• The mtDNA genome sequence may not the same in all mitochondria
• The phenotype reflects the proportion of mitochondria bearing the mutation